What is the difference in ESRI products between DLGs, shape files, and coverage files?
Only the "coverages" are specifically ESRI products:
DLGs are USGS "Digital Line Graphics". They used to be in a special vector format but now are often available in an SDTS format. See http://edc.usgs.gov/products/map/dlg.html .
Shapefiles were invented by ESRI as a kind of vector "lingua franca" for interchange of vector GIS data. "Shapefile" actually is a misnomer; the format consists of a collection of three files: a dBase-format file for the attributes, a well-documented binary format for the shapes, and a third file to map the attributes to the shapes. There is no explicit information about topology, projection, or any other metadata. Search www.esri.com for their shapefile format "white paper." A lot of software now reads and writes shapefiles directly. These files are fairly simple to read and write using C or some other general-purpose language.
Coverages are proprietary collections of files containing ArcInfo data. A coverage potentially contains explicit information about topology, projection, annotation, and other stuff. Their formats are not officially documented. Some third-party products can convert coverages into other formats. (I believe FME can do it.) This method of storing data is a genuine antique and is destined to fade rapidly now that ESRI has introduced its "Geodatabase."
--Bill Huber
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